After an oil drilling rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit, or workover rig, is typically used to service the well. Servicing includes, for example, installing and removing inner tubing strings, sucker rods, and pumps. This is generally done with a cable hoist system that includes a traveling block that raises and lowers the aforementioned tubing strings, sucker rods, and pumps.
U.S. Pat. No. 4,334,217 describes a system for monitoring the movement of a travelling block on a drilling rig. As described in the '217 patent, the traveling block can be raised or lowered beyond a safe limit. This is called “crown out” if the traveling block reaches its upper most safe position, and “floor out” if it reaches its lower most safe is position. Crown out/floor out can result in equipment damage and/or present a hazard to personnel working on the equipment. Because it is often not possible for the operator of the cable hoist system to see the position of the traveling block, or because the operator can be otherwise distracted from the position of the traveling block, the operator can inadvertently exceed safe positions of the traveling block.
The '217 patent identified the problem of unsafe hoist operation, and proposed a solution in which the total distance traveled by the traveling block is measured, and then compared with a reference point, such as the uppermost (crown) and lowermost (floor) position, of the traveling block. An electronic system was provided for displaying the position of the traveling block to the operator of the hoist system. In the event the operator failed to stop the traveling block from exceeding its uppermost and lowermost position, the system automatically switched off the hoist equipment if those limits were exceeded.
Although the '217 patent set out to solve the problem of unsafe hoist operation in an oil drilling rig, many drawbacks still remain when applying the '217 patent technology to a workover rig. For instance, hoist systems of workover rigs are much faster than those in oil drilling rigs, and the '217 system is not responsive enough to prevent the faster moving traveling block from crowning out or flooring out. Furthermore, the automatic switch-off system of the '217 patent provides for an abrupt stopping of the hoist system and traveling block. Abrupt stopping can cause an unsafe condition during workover operations and can possibly cause equipment damage, as the traveling block often supports a large amount of weight, often in excess of 100,000 pounds.